I didn't know that about the young children.
When my mother passed away when I was nine, I borrowed and wore my childhood best friend's very pretty satin red dress with white "fake fur" along the bottom of the dress. I thought that dress was real pretty, and I felt that my mother would have wanted me to wear something that I liked, instead of something plain and black. No one told me I had to wear black for the funeral, and no one told me what to wear. I just went over to my friend's house and asked her if she would mind if I borrowed her red dress for the funeral, and she let me wear it (she came to the funeral with me also because her mother was best friends with my mother). I saw no point in wearing black for a funeral. I am pretty sure my mother would have wanted me to wear something that I thought was very nice and pretty instead of drabby black clothes for her funeral.
I actually did not know that people were supposed to wear black to funerals until about a year later, and when I learned that, I went WTF. Most people who came to my mother's funeral wore a wide variety of bright or light colors of nice clothes. I don't remember much black on that day.
When I get old and die of old age, I do NOT want anyone to dress in all black for my funeral just because it is "proper". I want people to wear whatever they want/like to wear for the funeral.